As the Internet grows in popularity as a business medium, users engage in a wider variety of transactions online. Some of these transactions, such as transactions with financial institutions or online retailers, can involve sensitive personal information such as bank account numbers or credit card information, as well as access to money or other valuable resources. To control access to such information and resources, a variety of methods can be employed. For example, many online institutions require users to register with the institution and obtain a unique user name and password prior to transacting any business online.
One consequence of this system is that a username and password can only help protect the personal information of a user so long as the username and password remain under the control of the user. A third party who obtains the username and password of another can use that username and password to perform any transaction that the owner could perform. This consequence has spawned a practice commonly referred to as phishing.
Phishing can generally be described as an attempt by a third party to deceive a user into disclosing his username and password to that third party through the third party's impersonation of an entity that is known and trusted by the user. Generally, a phishing attack can be initiated by sending an electronic mail message to a user that is crafted to appear to originate from a known and trusted entity. Such electronic mail messages commonly inform the recipient that the entity must verify the information of the user by having the user enter his username and password. The user may enter this information at a web site that appears to belong to the known and trusted entity but is actually controlled by a third party. Once the user enters this information at the web site of the third party, sometimes called a phishing site, the third party can use the entered username and password at the real website of the entity that the third party is impersonating to perform transactions or even to wrest control of an account with the known and trusted party away from the user.
Several factors make phishing a challenging problem from a computer security standpoint. First, in phishing attacks the victim unknowingly or unwittingly assists the attacker by voluntarily providing his security credentials such as a username and password to the attacker. Second, identifying phishing sites can be difficult using a fixed algorithm because attackers both quickly adapt to security measures and it is difficult if not impossible to anticipate the ingenuity of all future attackers with a fixed set of rules. Third, users tend to ignore warnings about security dangers. Even the best warnings can be rendered useless by a user who does not heed the warning.